Recently, there are increasing demands for further refinement of circuit patterns for increasing the degree of integration of a large scale integrated circuit device (hereinafter referred to as the LSI) realized by using semiconductor. As a result, it has become very significant to thin an interconnect pattern included in a circuit.
Now, the thinning of an interconnect pattern by a conventional optical exposure system will be described on the assumption that positive resist process is employed. In this case, a line pattern means a portion of a resist film not exposed to exposing light, namely, a resist portion (a resist pattern) remaining after development. Also, a space pattern means a portion of the resist film exposed to the exposing light, namely, an opening portion (a resist removal pattern) formed by removing the resist through development. In the case where negative resist process is employed instead of the positive resist process, the definitions of the line pattern and the space pattern are replaced with each other.
When a pattern is formed by using the optical exposure system, a photomask in which a completely shielding pattern of Cr (chromium) or the like is drawn in accordance with a desired pattern on a transparent substrate (a permeable substrate) of quartz or the like is conventionally used. In such a photomask, a region where the Cr pattern exists is a shielding portion that does not transmit exposing light of a given wavelength at all (having transmittance of substantially 0%) and a region where no Cr pattern exists (an opening) is a transparent portion that has transmittance equivalent to that of the transparent substrate against the exposing light (having transmittance of substantially 100%). In the case where a wafer on which a resist is applied is subjected to exposure by using this photomask, the shielding portion corresponds to an unexposed portion of the resist and the opening (the transparent portion) corresponds to an exposed portion of the resist. Accordingly, such a photomask, namely, a photomask composed of a shielding portion and a transparent portion against exposing light of a given wavelength, is designated as a binary mask.
It is, however, difficult to form a fine pattern smaller than the exposure wavelength (the wavelength of the exposing light) by using the binary mask due to the diffraction phenomenon of light. Therefore, a mask pattern having a function to invert the phase of light, namely, a photomask provided with a phase shifter, is recently used. Furthermore, as a photomask for largely increasing the contrast and DOF (depth of focus) in fine pattern formation, a mask having a mask enhancer structure (an image enhancement mask) devised by the present inventor may be used in pattern formation. The mask enhancer structure basically includes a phase shifter and a shielding pattern such as a Cr pattern (see, for example, International Application PCT/JP00/07772 laid open in accordance with Patent Cooperation Treaty (International Publication No. WO 01/35166 A1), hereinafter referred to as Literature 1).
FIG. 19A shows an exemplified plane structure of an image enhancement mask, and FIG. 19B shows light amplitude intensity obtained through exposure of the mask of FIG. 19A on a material to be exposed in a position corresponding to a line AA′.
As shown in FIGS. 19A and 19B, a transparent phase shifter is provided within a pattern of a shielding film such as a Cr film (a shielding portion) in the image enhancement mask, so that the contrast in a light intensity distribution formed in the exposure can be enhanced.
The image enhancement mask of FIG. 19A is obtained by combining a mask shown in FIG. 19C in which a shielding portion (a shielding pattern) is surrounded with a transparent portion and a mask shown in FIG. 19D in which a phase shifter is surrounded with a shielding portion. FIG. 19E shows the light amplitude intensity obtained through the exposure of the masks of FIGS. 19C and 19D on materials to be exposed in positions corresponding to lines AA′.
As shown in FIGS. 19C and 19E, in the light intensity distribution formed in the case where a mask including a line-shaped shielding portion (specifically a Cr pattern) alone is used, as the line width of the Cr pattern is smaller, the light intensity obtained at the center of the Cr pattern (namely, the mask pattern center) is increased owing to light rounding the periphery of the Cr pattern. In other words, it cannot sufficiently shield the light although it is a shielding pattern. Accordingly, a sufficient shielding property cannot be realized, resulting in lowering the contrast in the light intensity distribution.
On the other hand, as shown in FIGS. 19D and 19E, in the case where a phase shifter for transmitting light in an opposite phase with respect to the transparent portion is used, if light passing through the phase shifter and light passing through the transparent portion can be made interfere with each other, these light cancel each other. Accordingly, with respect to a pattern having a line width too small to sufficiently shield light by using a Cr film, when a transparent phase shifter is formed within the Cr pattern, light passing through the phase shifter can be made to interfere with light passing through the transparent portion disposed around the Cr pattern and rounding to the back side of the Cr pattern, resulting in realizing a mask pattern having a very high shielding property. Such a structure in which a phase shifter for canceling light passing through a transparent portion is provided within a mask pattern with a shielding property is designated as a mask enhancer structure.
Although the mask enhancer structure is described to be composed of a shielding film such as a Cr film and a phase shifter in the above description, the equivalent effect can be attained when the shielding film is replaced with a semi-shielding film. A semi-shielding film partially transmits light, and light passing through the semi-shielding film and light passing through the transparent portion are in an identical phase.
Also, in pattern formation using a mask having the mask enhancer structure, the contrast in the light intensity distribution is increased/reduced as well as a process margin is increased/reduced in accordance with the combination of the width of the Cr film and the width of the phase shifter. The aforementioned Literature 1 discloses, as a method for increasing a process margin in formation of an isolated pattern, that a thin phase shifter is provided within a thick Cr pattern and a thick phase shifter is provided within a thin Cr pattern.
In the case where the mask enhancer structure is employed in a photomask including an isolated pattern alone, a large process margin can be attained by providing a thin phase shifter within a thick Cr pattern and a thick phase shifter within a thin Cr pattern as described above. However, in the case where the mask enhancer structure is employed in a photomask not only including an isolated pattern but also having complicated patterns mixedly having arbitrary pattern layouts, the process margin cannot be sufficiently increased simply by changing the width of the phase shifter in accordance with the width of the Cr pattern.